Page 174 - CW E-Magazine (2-4-2024)

Special Report

pellers, valve bodies, condensers/heat- stress on the components by stress- and where relatively high fabrication
exchangers, water-boxes, radiators, and relieving heat treatments, or by shot- costs are involved. It may be necessary
propellers, etc. for minimising erosion- peening the surface to introduce com- to compromise and sacrifice some
corrosion effect. pressive stresses. Corrosion fatigue mechanically advantageous properties
resistance can also be increased by to satisfy corrosion requirements and
Fretting corrosion using coatings such as electrodeposited vice versa.
Fretting corrosion can be prevented zinc, chromium. Nickel, copper, and
by selecting the following combination nitride. When electrodeposited coat- Where corrosion rate is either very
of materials: ings are applied it is important to use low or very high, the choice of mate-
Laminated plastic on gold plate; plating techniques that do not produce rials is simple; where it is moderately
Hard tool steel on tool steel; tensile stresses in the coating or charge low, a thorough analysis of all aspects
Cold-rolled steel on cold-rolled steel; hydrogen into the metal. is required.
Cast iron on cast iron with phos-
phate coating; coating of rubber Hydrogen blistering and embrittlement In dry environments and carefully
cement, tungsten sulphide; Use of ‘clean’ steel greatly increases controlled fluids, many materials can
Cast iron on cast iron with rubber resistance to hydrogen blistering be used, and these often may be left
gasket; because of absence of voids in the mate- unprotected. Under atmospheric con-
Cast iron on cast iron with Moly- rial. Nickel containing steels and nickel ditions, even polluted atmospheres,
kote lubricant; and base alloys having very low hydrogen metals as SS and aluminium alloys
Cast iron on SS with Molykote diffusion rates are often used to prevent may be left unprotected. Also, copper
lubricant. hydrogen blistering. Metallic, inorga- and lead have a long life. In a more
nic, and organic coatings and liners are severe wet environment, for example in
Stress corrosion cracking often used to prevent hydrogen blister- marine conditions, it is generally more
The residual stress in the material ing of steel containers. However, the economic to use relatively cheap struc-
may be removed by annealing or the coating or liner must be impervious to tural materials (mild steel) and apply
section subjected to residual stress may hydrogen penetration and be resistant additional protection, rather than use
be thickened. Plain carbon steel may be to the mediums contained in the tank. the more expensive ones. For severest
stress-relief annealed at 1100-1200°F, Steel clad with austenitic SS or nickel corrosive conditions it is preferable in
and the austenitic SS are frequently is often used for this purpose. Also, most cases to use materials resistant to
stress relieved at temperatures ranging rubber and plastic coatings and brick the corrosive than to use cheaper mate-
from 1500-1700°F. SCC takes place for linings are frequently used. Baking of rials with an expensive protection.
certain alloys in some specific environ- steels at low temperatures (200-300°F)
ments and hence should not be used for removing hydrogen may prevent Materials more expensive than
in those environments. Changing the hydrogen embrittlement. Low-hydro- absolutely necessary should not be chosen
alloy is one possible option if neither the gen welding rods should be used and unless it is economical in the long run
environment nor stress can be changed. dry conditions during welding should and necessary for the safety of person-
For example, it is common practice to be maintained to prevent hydrogen em- nel or product or for other important
use Inconel (raising the nickel content) brittlement. reasons. Using fully corrosion resis-
when type 304 SS is not satisfactory. tant materials is not always the correct
Although carbon steel is less resistant General guidelines for selection of choice – a balance between first cost
to general corrosion, it is more resistant materials(4, 5): and cost of subsequent maintenance
to SCC than SS. Thus, under conditions Materials should be selected with should be found over the full estimated
which tend to produce SCC, carbon due consideration to their functional life of the designed utility.
steels are often found to be more satis- suitability and ability to maintain their
factory than SS. For example, heat function safely for an economical Composition of alloy alone does not
exchangers used in contact with sea- period of time at a reasonable cost. The ensure quality of the product. Evalua-
water or brackish waters are often whole material complex should be con- tion of resistance to corrosion in a
constructed of mild steel. sidered as an integrated entity, rather given environment, adverse effect of
than each material separately. The corrosion products on utility or con-
Corrosion fatigue more highly resistant materials should tents, susceptibility to a specific type of
It can be prevented by reducing the be chosen for the critical components corrosion and fouling, and tendency to

174 Chemical Weekly April 2, 2024

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